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Characteristics of Numerically Simulated Mesoscale Convective Systems and Their Application to Parameterization

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  • 1 The Met. Office, Bracknell, Berkshire, United Kingdom
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Abstract

A cloud-resolving model simulation of a convectively active phase of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) is used to study the properties of the convective and mesoscale updrafts, and precipitating downdrafts of the simulated mesoscale convective systems (MCSs). Analysis of scalar transports confirms the results of previous studies that show that the mesoscale updraft provides an important contribution to the total heat and moisture budget of the MCS in the upper troposphere. The mesoscale updraft is shown to possess little or no positive buoyancy, and it is suggested that the upward vertical motion present is associated with decaying convective cells. Momentum transports are not significant in the mesoscale updraft, but those in the mesoscale downdraft are of the same order as those in the convective downdraft. Horizontal mesoscale circulations, such as rear-to-front inflow, may be important in determining the downdraft momentum transport.

Using the cloud-resolving model results, a mass flux parameterization for the mesoscale updraft is proposed. The mass flux equation is closed by assuming that a fraction of the mass flux detrained from the convective cores is entrained into the mesoscale updraft. Along with a simple evaporation-based parameterization for the mesoscale downdraft, the parameterization is tested in a single-column model for the same TOGA COARE period. Comparisons of the cloud-resolving model and single-column model results show very good agreement between the simulated and parameterized mesoscale scalar transports when the parameterized convection is also well represented. The importance of an accurate convection scheme to drive the mesoscale updraft–downdraft parameterization is stressed.

Corresponding author address: Dr. M. E. B. Gray, The Met. Office, London Road, Bracknell, Berkshire RG12 2SZ, United Kingdom.

Email: mebgray@meto.gov.uk

Abstract

A cloud-resolving model simulation of a convectively active phase of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) is used to study the properties of the convective and mesoscale updrafts, and precipitating downdrafts of the simulated mesoscale convective systems (MCSs). Analysis of scalar transports confirms the results of previous studies that show that the mesoscale updraft provides an important contribution to the total heat and moisture budget of the MCS in the upper troposphere. The mesoscale updraft is shown to possess little or no positive buoyancy, and it is suggested that the upward vertical motion present is associated with decaying convective cells. Momentum transports are not significant in the mesoscale updraft, but those in the mesoscale downdraft are of the same order as those in the convective downdraft. Horizontal mesoscale circulations, such as rear-to-front inflow, may be important in determining the downdraft momentum transport.

Using the cloud-resolving model results, a mass flux parameterization for the mesoscale updraft is proposed. The mass flux equation is closed by assuming that a fraction of the mass flux detrained from the convective cores is entrained into the mesoscale updraft. Along with a simple evaporation-based parameterization for the mesoscale downdraft, the parameterization is tested in a single-column model for the same TOGA COARE period. Comparisons of the cloud-resolving model and single-column model results show very good agreement between the simulated and parameterized mesoscale scalar transports when the parameterized convection is also well represented. The importance of an accurate convection scheme to drive the mesoscale updraft–downdraft parameterization is stressed.

Corresponding author address: Dr. M. E. B. Gray, The Met. Office, London Road, Bracknell, Berkshire RG12 2SZ, United Kingdom.

Email: mebgray@meto.gov.uk

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